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Forces Notes
“Any influence that can cause a body to be
It is measured in
accelerated
.
.”
Newtons
A
Newton
is the force needed to accelerate one kg one meter per second per second
A
Push or
Pull exerted by an object to/on another object
Always occurs in
It can alter the
Pairs
(Sometimes referred to as Agent & Receiver)
Shape
It is represented by
or
Arrows
Motion
that show
Demos
of an object
Direction
and size
What it demonstrates
Rolling Ball
Shows Acceleration
Pushing a closed door
All show that forces occur in pairs
Standing on the floor
All show that forces occur in pairs
Clay ball
Shows how force is required to change shape
Tennis ball
Change of shape- Elasticity
Pushing on a table
Forces occur in pairs; Net Force
Newton’s Laws of Motion
Law #1:
“Every body continues in its state
of
rest
, or of
uniform motion in a straight line,
unless it is compelled to
by forces impressed upon it.”
Sometimes called the Law of
Change
that state
Inertia
Who is this? Sir Isaac Newton
What is Inertia?
“The sluggishness or apparent Resistance
a body offers to changes in its state of motion”
The larger the
Mass
A body with less
, the larger its
Mass
A body with more mass is
Inertia
is easier to get moving
harder
to get moving
Demos
What it demonstrates
Ping pong ball
Small inertia; easy to move & stop
Bowling ball
Larger inertia; hard to move & stop
Granny
Small inertia; easy to move & stop
Mack Truck driven by Granny 
Larger inertia; hard to move & stop
Mass
Defined as “The quantity of
Matter
It is the measurement of
Inertia
in a body”
m = mass; F = force; a = acceleration
m
= F/a
F
= m(a)
a
Why should Eeyore ALWAYS wear a
seatbelt?
= F/m
Momentum
The product of
It is
Mass
Inertia
and
in motion
Demos
Car on a ramp
Velocity
What it demonstrates
A small mass accelerated at high velocity can be produce a lot of force
Eeyore on a car on a ramp
Momentum
Conservation of Momentum
All energy stored in a moving object is not lost
Stated by:
Albert Einstein
, but only
changes
Phillip Morrison
Demos
form
Mr. Watts
What it demonstrates
Newton’s Cradle
Conservation of Momentum
Swinging Rock
Potential and Kinetic energies; Conservation of Momentum
Mass in a Circular Motion
When an object moves in circular path, it has a property called
AKA:
Gyroscopic
Rotational
Inertia
Inertia
“An object rotating about an axis tends to remain rotating about that same axis unless it is
interfered with by some external force.”
This definition is similar to Newton’s
First Law of Motion
Demos
What it demonstrates
Bicycle tire
Gyroscope
Top
Frisbee
Football
Moment of Inertia
Angular Momentum
It is a measure of an object’s rotation
about a particular
Axis
For an object small compared to the radial distance, it is
the product of mass, velocity and radius (m x v x r)
It is the measure of the rotational property
of
motion
It is
perpendicular
the centripetal force
Describe what’s happening in this diagram:
to
Demos
What it demonstrates
Rolling objects
Washer on a string
Slingshot (old-fashioned!)
Bucket with water spun in vertical circles
Q: What holds the water in the bucket?
Spin cycle of washing machine
“Whizzer”
Brendan on the Scrambler
Carnival Ride Know-How
It’s better to take the
outside
of the car if you’re riding alone
If you’re riding with someone
else, take the
inside
seat
seat
The following formula is similar to those
for Newton’s
First Law
, but
deals with mass in motion in a circular path: mvr = mvr
m=
Mass
v=
Velocity
r=
radius
What was demonstrated to show what happened
here?
What happens in this situation?
mv
r = mvr
… The mass is not affected by a smaller radius, while
is.
Centripetal Force
A center-seeking
Force
that causes an object to follow a circular path.
“Any force that is directed at
produces circular motion.”
right angles
to the path of the moving body and
Name the centripetal force that keeps the moon
revolving around the earth:
Centrifugal Force
A fictitious
due to rotation
outward
force
It is a
Reaction
to centripetal
force but there is no agent, therefore it is experienced
relative to position
What we are really experiencing is
ANGULAR MOMENTUM
!
Newton’s Second Law
The acceleration of a body is
and
inversely
NET
FORCE
Formulas:
a
= F/m
F
= m (a)
m
= F/a
directly
proportional to the net force acting on the body
proportional to the mass of the body and is in the direction of the
5N
5N
5N
Acceleration = 6 kph
Acceleration = _____
Acceleration = _____
N
Newton’s Third Law
Whenever one body exerts a
force
on a second body, the
second body exerts an
equal
and opposite force on the first
For every
action
, there is an equal and opposite reaction
You cannot
touch
without being
touched
Common Forces
Gravitational – non contact
5 Every object exerts an
N
Buoyancy - contact
The tendency to
Acceler
ation = ?
attractive
force on every other object
float
in a fluid
Friction - contact
The
resistance
to motion where two objects touch
Magnetic – non contact
The repelling or
Attraction
of a magnet and ferromagnetic material
Elastic - contact
Ability to
spring back
to its original size, shape, or position after being
stretched, squeezed, flexed, expanded, etc.
Electrical – non contact
Flow or gathering of a negative charge by
Other Physics concepts of interest:
Energy
The quality of an object that enables it to do
electrons
K
P
K
work
Power
Work/ Time
Potential Energy
The stored
energy that a body
possesses because of its position with respect to other bodies
Kinetic Energy
The energy of
motion
Kinetic Energy = 1/2 mv2
P
At which point is potential and kinetic
energy equal?